Electric fan relay ground bypass

[EDZIP]

Wiring up a 1995 Camaro LT1 engine in a streetrod...might be a basic question...but can I safely make an ECM ground signal bypass switch for Cooling Fans 1&2 relays without causing damage to the ecm?

Just in case we want the fans on early in traffic or if for some reason the ecm signals fail.

Thanks

[RobertISaar]

so, a secondary method of activating the fans? doable. if you tap into the existing relay wiring, you'll likely set a SES code for a quad driver, but if you were to add a second relay and have it triggered only via a manual switch, that would allow both scenarios without causing a SES. you won't get the instant IAC step addition that the ECM would perform due to increased alternator load, but it would dial it in after the idle speed sags. for the same reason, if/when you turn it off, you'll get some overshoot as well.

[Six_Shooter]

I've added a switch to fan circuits several times without additional parts and never got an SES light. I don't think the ECM checks for output when not triggered.

If you're really worried you could use a diode to isolate the ECM from the signal from the switch.

[RobertISaar]

depends on the ECM.... some don't have the quad/output driver error indication circuits in any way connected, others don't implement setting the SES in the software and just set a status flag instead.